The issue is that NiCad and Lithium-Ion cells are 1.2v each, FX Headlight, FX Ion and FX3's all have 3 cells = 3.6v. I dont know what voltage the 7 LED arrays are designed to run at, but if it is 3.6v, then using a 4.5 volt flatpack or 4.4 volt Oldham (lead acid =2.2v per cell) will over-run them by 0.8-0.9v. I have heard that over-running LED's shortens thier life but I do not know how much you have to over-run them to have a significant effect.
If you are worried about it you can fit an adapter into the battery pack, petzle do one, and use three AA NiCad's. This will shorten run time, each AA NiCad rechargable cell is normally 300mAh x3=0.9Ah, 1amp bulb will run for 0.9 hours, 0.5 amp bulb will run for 1.8 hours. A way around this is to use more industrial NiCad's with a greater duration.
When all is said and done a little it of thought and fiddling with Oldham headsets, LED's and battery packs gets you a well tough useable light solution and is better than spending £100, £170, for a badly made Headlight or FXIon.